This invention relates to aromatic polyimides and, more particularly, to blends comprising novel imide copolymers and polyether sulfones. The blends are miscible, and display excellent mechanical properties combined with high thermal stability, low water absorption and good melt-fabricability.
Aromatic polyimides have been known for over 30 years, and a variety of imide-containing polymers are readily available from commercial sources. Polyimides are generally recognized as excellent in heat resistance characteristics, but difficult to process. Though linear, polyimides do not behave as thermoplastics and melt-processing using injection molding or extrusion is difficult if not impossible for most members of this class of polymers. Fabrication of wholly aromatic polyimide molding powders may be accomplished in most instances only by sintering at high temperature and pressure, using techniques and procedures analogous to those used in powder metallurgy.
Solution processes such as dope casting also have very limited application to the processing of polyimides due to the characteristic insolubility of wholly aromatic polyimides. Where solution fabrication of polyimides is desired, such as for the production of coatings and film, the process generally carried out with the soluble polyamide-acid precursor, i.e., the form of the polymer prior to imidization. The subsequent step of thermally-inducing ring closure of polyamide-acids in the fabricated article to provide the polyimide is accompanied by evolution of water, making the fabrication of void-free articles extremely difficult and tedious.
Polyimides described as thermoplastic are also known including, for example, polyimides derived from 1,4-bis(p-aminocumyl) benzene, disclosed broadly in U.S. Pat. No. 4,681,928. Tetramethyl analogs of the polyimide are also known in the art. Such polyimides are described as soluble, having high glass transition temperatures and good mechanical and thermal properties.
Imide-based resins derived from diphenyl sulfone tetracarboxylic acid dianhydride, also called sulfonyl bis(phthalic anhydride) or "SPAN", are also known in the art, and are described in, for example, U.S. Pat. No. 3,422,061.
Polyimides having isopropylidene linkages have been described in the art and, more recently, blends of such polyimides with poly(aryl ether sulfones) or PAES have also been described, for example, in commonly assigned U.S. Pat. No. 5,037,902. The disclosure of the patent also includes polyimides derived from sulfonyl bis(phthalic anahydride) and 1,4-bis(p-aminocumyl) benzene.
Polyimides, including those described in the art as melt-processable, are known to exhibit some shortcomings. Seeking to overcome the deficiencies, the art has turned to condiseration of blends comprising two or more thermoplastics. These efforts have resulted in identifying blends having a unique balance of properties not achievable by a single polymer, and such blends have found wide commercial application. Most polymers are incompatible, and although blends of incompatible polymers may have utility for many uses, blends comprising compatible and preferably miscible polymers generally exhibit more desirable mechanical properties, improved optical clarity and physical uniformity, together with better thermal performance and improved processing characteristics. Such blends often find wider acceptance and, where adequate compatibility is lacking, the art has often sought additives to be included with the polymeric components to act as compatibilizing agents, enhancing compatibility of the polymeric components and in turn further improving important blend properties.
Compatibility and miscibility with other polymers, together termed "alloyability" are thus clearly considered by the art to be desirable polymer characteristics, and alloyability is capable of conferring a definite commercial advantage for such resins. In U.S. Pat. No. 5,037,902 there are described blends comprising poly(aryl ether sulfones) and certain polyimides derived from 1,4-bis(p-aminocumyl) benzene. According to the patent, only poly(aryl ether sulfones) comprising biphenyl units are generally miscible with polyimides derived from 1,4-bis(p-aminocumyl)benzene, and the presence of biphenyl units in the poly(aryl ether sulfone) unit is said to therefore be critical to attaining miscibility over a wide range. The patent defines miscibility as the presence of a single Tg for the blend, intermediate between the Tg values for the individual components, and recognizes that some combinations may be partially miscible, defined as miscibility of the two resin components over less than the entire range of compositions. Resin formulations comprising these blends have good high temperature properties while remaining thermally processable. However, such blends are lacking in toughness, particularly in impact properties, and their acceptability for many applications will thus be somewhat limited. In addition, biphenyl-containing poly(aryl ether sulfone) resins are less readily available than many sulfone-containing resins, and may be more costly, factors that somewhat limit their commercial acceptance. Polyether sulfone resins are readily available from commercial sources. These resins are typically more readily processable than the corresponding biphenyl-containing poly(aryl ether sulfone) resins and are substantially lower in cost, yet have the toughness and chemical resistance needed for use in a wide variety of applications. Although polyether sulfones have a high Tg of about 220.degree. C., they may be somewhat lacking in high temperature properties, and their acceptability for use under more demanding environmental conditions may be somewhat limited as a result. A method for extending the temperature range in which polyether sulfones may be used could increase the acceptance of such materials for use in extreme environments, particularly where the chemical resistance of sulfone-containing polymers is needed. Compositions comprising polyether sulfones which are useful over a widened temperature range and have improved strength and rigidity while retaining the good processability of the polyether sulfones would clearly represent a useful advance in the art.